check return value of verify_ref_format()

check return value of verify_ref_format()

Users of the ref-filter code must call verify_ref_format()
before formatting any refs, but most ignore its return
value. This means we may print an error on a syntactically
bogus pattern, but keep going anyway.

In most cases this results in a fatal error when we actually
try to format a ref. But if you have no refs to show at all,
then the behavior is confusing: git prints the error from
verify_ref_format(), then exits with code 0 without showing
any output.  Let's instead abort immediately if we know we
have a bogus format.

We'll output the usage information if we have it handy (just
like the existing call in cmd_for_each_ref() does), and
otherwise just die().

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>